Control elements of low foaming hollow blow molding
DATE：2021/10/18 8:31:59 / READ： / SOURCE：This station
Control elements of low foaming hollow blow molding
The reason why the commercialization of hollow blown foam is difficult is because the cells are difficult to control during the inflation process. Many published research results just stay at the laboratory level. Based on the successful production experience and research results of a few companies, the following control elements can be summarized.
①Chemical blowing agent can usually be uniformly premixed into the raw materials for hollow blow molding. In this way, it is easy to ensure the uniformity of the foaming agent distribution in the plastic, thereby also ensuring the uniform distribution of the final product.
②The better position for inert gas injection into the plastic melt should be as close to the machine head as possible. Therefore, when inert gas is injected through the screw, the injection port should be at the head of the screw. In order to make the gas evenly distributed in the plastic melt, the inert gas injection port should be located off the center of the screw head to achieve a certain stirring effect by means of the rotation of the screw itself.
③As the inflation process is inevitable for hollow blow molding, all hollow blow molded foam plastic products have varying degrees of cell deformation (lengthening, flattening). However, if the degree of bubble deformation can be controlled within a certain limit, the final quality of the hollow blow molded parts can be guaranteed to be within an acceptable range. Therefore, the design of low-foaming hollow blow molding parts is required to avoid excessively thin walls. Generally, large bubbles are most likely to occur at the thin wall of the workpiece. Such large bubbles are elongated too much in the process of inflation and stretching and rupture and collapse, resulting in insufficient strength of waste products or parts.
④ When using the extrusion method to make billets, pay attention to controlling the extrusion speed. The general relationship is that the faster the extrusion speed, the larger the cells obtained, the larger the cells, and the greater the possibility of excessive cell deformation or cell rupture during the inflation process.
⑤In the hollow molding, the relationship between the inflation pressure and the density of the foam must be taken into account (see Figure 10-5). It can be seen from the figure that if the inflation pressure is high, the bubbles will be destroyed more and the expansion ratio will decrease (or the density of the foam will increase). If the inflation pressure is too low, the reproducibility of the cavity shape will be poor. Therefore, under the premise of satisfying the reproducibility of the cavity shape, the inflation pressure should be as low as possible.
⑥The surface quality of low-foaming hollow blow-molded parts can be improved by rapidly cooling the mold and reducing the blowing pressure. Generally, under good mold cooling conditions, the inflation pressure can be between 0.3 and 1.05 MPa depending on the material, size and shape of the mold cavity.
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